Our studies began with the observation of a marked increase in metastasis incidence when tumor cells (402AX teratocarcinoma) were grown in metastasis-permissive hosts. Such a host could be produced by the transfer of an early whole mouse embryo (Day 7) into the peritoneal cavity. The positive effect of such embryonic allografts only occurred when they were surgically placed in the peritoneal cavity (subcutaneous grafts had no effect). A hypothesis was formulated based on this observation: the interaction of the embryo and the host-peritoneal cells produced a factor(s) that affected tumor growth and spread in a positive way. This concept was supported by the in vitro observations that media conditioned by organ cultures of early mouse embryos had no effect on the growth and metastasis of pretreated tumor cells. In contrast, media conditioned by organ cultures of embryos grown on feeder layers of peritoneal cells had a marked effect on metastasis production. Further studies have shown that the embryos could be replaced by estrogen. When monolayers of peritoneal cells (mesothelial cells, stroma, and macrophages) are pretreated with estrogen, they increase the protein concentration of the media. Such conditioned media has mitogenic activity for 402AX teratocarcinoma cells in vitro and enhances tumor growth and metastasis in vivo. Efforts to identify these putative growth factors/modulators from conditioned media have been difficult, due mainly to the large amounts of media that are required. Recently we have begun whole organ extractions of mouse tissues treated with estrogen. This approach appears to be very promising. Partially purified extracts of the peritoneal and uterine cavities contain peptides that are mitogenic towards tumor cells in vitro and in vivo. Further work is required to characterize and identify these factors. (A)